1. Field of the Invention
The present invention relates to a photomask used in an apparatus for manufacturing a semiconductor integrated circuit and to a method which uses this photomask to expose the pattern of a semiconductor integrated circuit.
More specifically, it relates to a mask for forming the contact holes of a memory device and to a contact hole exposure method.
2. Description of Related Art
In the past, the increasing level of integration of semiconductor integrated circuits has resulted in a rapid shrinking of circuit patterns used therein. However, the lithography technology that uses a projection-type exposure apparatus has a limit of resolution that is related to the wavelength of the light source.
In recent years, there have been various methods proposed for achieving ultra-high resolution, such as the method of using a phase-shift mask or using illumination of a particular shape to improve the resolution.
One known half-tone phase-shift mask for the purpose of improving the resolution for hole patterns, is known by its disclosure in the Japanese Unexamined Patent Publication (KOKAI) No. 4-136854. With a half-tone phase-shift mask, a translucent phase-shift mask is used in place of the usual light-blocking film.
However, when a half-tone phase-shift mask is used, because of the very small amount of light that passes through the translucent phase-shift mask, there is a region of strong light intensity surrounding the hole pattern, this being known as a side lobe.
In the case of a memory device, therefore, in which the spacing between contact holes is narrow, as the size of features becomes smaller, these side lobes overlap with one another, so that unwanted side lobes are transferred onto the resist.
Another known method of improving the resolution for hole patterns other than the use of a half-tone phase-shift mask is that, for example, which is disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 4-268714.
This method, as shown in FIG. 6, makes use of a mask onto which are provided auxiliary holes 5, which are below the resolving limit of the projection lens, in the vicinity of the area surrounding the hole pattern 4.
By exposing through this mask using a ring-shaped light source 10 as shown in FIG. 5(A) or a four-point light source 11 as shown in FIG. 5(B), it is possible to improve the hole resolution.
These types of masks are effective for use with a hole pattern of sparsely distributed isolated holes.
Another method is that which is disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 62-67514, in which is presented a proposal, as shown in FIG. 7, of a mask which is provided with auxiliary shifters 9, which surround the hole pattern 4, and which shift the phase of the light passing therethrough by 180 degrees, these being of a size that is smaller than the resolving limit of the projection lens.
This mask imparts a phase shift effect to isolated holes, and improves the resolution when used with illumination having a small numerical aperture .sigma. of 0.3 or smaller in an illumination optical system.
A mask with auxiliary holes or auxiliary shifters such as shown in FIG. 6 and FIG. 7 is effective with respect to isolated holes.
However, the arrangement of holes in the contact hole pattern of a memory device, such as shown in FIG. 3, has parts in which the holes are arranged in high concentration.
For example, in the case of the capacitive contact hole that is shown in FIG. 3(C), it is not possible to provide auxiliary holes or auxiliary shifters that surround the hole pattern 4.
Accordingly, it is an object of the present invention to provide a photomask that alleviates the above-noted problems which accompanied the prior art and which improves the resolution of a contact hole pattern of a memory device, and to provide an exposure method which uses this photomask.